The difference is that a leaving leader even breaks in the happy case: if any other node leaves, everything is just fine, but if it happens to be the leader then these three random gossips are all that stand between “normal function” and “needing operator intervention”, and the distinction is rather arbitrary (as any node could be the leader). If the new leader would just detect convergence if the previous leader is LEAVING and everybody else has seen that, then the problem would be solved.